1. Field of the Invention
The present invention relates to an optical pickup used for reading information on an optical disc in a DVD recorder, a personal computer or the like.
2. Description of the Related Art
As image or audio recording media, optical discs such as a DVD (digital versatile disc) and a CD (compact disc) have come into widespread use today. Further, recently, new optical discs such as a Blu-ray Disc (trademark) and a HD-DVD (high definition digital versatile disc) capable of recording larger-capacity data have appeared.
In such a optical disc, reading information and servo control are performed by projecting laser beam with a predetermined wavelength on an information recording surface of the disc and receiving reflected beam of the laser beam. The wavelength of the laser beam varies in accordance with the species of optical discs. For example, infrared laser beam with a wavelength of 780 nm is used for a CD, red laser beam with a wavelength of 650 nm is used for a DVD, and blue laser beam with a wavelength of 410 nm is used for a BD (Blu-ray Disc) and a HD-DVD, respectively. Therefore, also in an optical pickup, compatibility with a plurality of media is required.
Japanese Unexamined Patent Publication No. 2004-139709 discloses an optical pickup including two laser beam sources so as to record and reproduce for both the CD and the DVD. Japanese Unexamined Patent Publication No. 2004-103135 discloses an optical pickup including three laser beam sources so as to record and reproduce for a BD in addition to a CD and a DVD.
Generally, an optical pickup is constructed so as to project laser beam from a light source on an information recording surface of an optical disc through a beam splitter and to receive beam reflected at the information recording surface by a light receiving section through the beam splitter. As the beam splitter, for example, a half mirror (hereinafter, also referred to as just a “mirror”) is used, and transmittance of the mirror depends on the wavelength of the laser beam and also depends on an incident angle of the laser beam into the mirror.
FIG. 6 shows graphs of an example of wavelength dependency and incident angle dependency of transmittance of a mirror. FIG. 7 is a view showing an example of a light receiving system for illustrating FIG. 6. FIG. 7 shows a half mirror 3, a collimating lens 4, and a light receiving section 8. Beam (only zero-dimensional beam Z is shown herein) projected from a light emitting device (not shown) and reflected at the optical disc is gathered to the light receiving section 8 through the collimating lens 4 and the half mirror 3. Since the reflected beam Z has a diameter of a beam of a certain width, when an incident angle of a central beam Z0 to the mirror 3 is defined as θ, an incident angle θ1 of an outer beam Z1 to the mirror 3 becomes larger than θ (θ1>θ), and an incident angle θ2 of an outer beam Z2 to the mirror 3 becomes smaller than θ (θ2<θ).
When θ1 and θ2 are set as θ1=θ+10° and θ2=θ−10°, respectively, variations in transmittance for the respective wavelengths are shown in FIG. 6. A solid line in FIG. 6 indicates transmittance of the central beam Z0 (incident angle θ), a dashed-dotted line indicates transmittance of the outer beam Z1 (incident angle θ1=θ+10°), and a broken line indicates transmittance of the outer beam Z2 (incident angle θ2=θ−10°).
In FIG. 6, in view of variations in transmittance in the wavelength of 780 nm corresponding to the CD, the wavelength of 650 nm corresponding to the DVD, and the wavelength of 410 nm corresponding to the BD (as well as the HD-DVD), in a case of 780 nm (for the CD), the variation of the dashed-dotted line (deviation from the solid line) is larger than that of the broken line, and therefore it is found that the variation in transmittance is large with respect to beam incident at an angle (θ1=θ+10°) larger than θ in this wavelength. On the other hand, in a case of 650 nm (for the DVD), variation of the broken line is larger than that of the dashed-dotted line, and therefore it is found that variation in transmittance is large with respect to beam incident at an angle (θ2=θ−10°) smaller than θ in this wavelength. Also in a case of 410 nm (for the BD and HD-DVD), variation of the broken line is larger than that of the dashed-dotted line, and therefore it is found that variation in transmittance is large with respect to beam incident at an angle (θ2=θ−10°) smaller than θ in this wavelength.
Thus, the transmittance of the mirror 3 varies with a wavelength of laser beam to be used and an incident angle of reflected beam to the mirror. However, there is no extreme difference in transmittance between the CD and the DVD as is evident from FIG. 6. On the other hand, when the BD (HD-DVD) and the CD/DVD are compared with each other, there is a large difference in transmittance. Therefore, in a two wavelength-compatible optical pickup compatible with only the CD and the DVD, effects by the wavelength dependency and the incident angle dependency of the transmittance are small, while in a three wavelength-compatible optical pickup compatible with the BD and the HD-DVD in addition to the CD and the DVD, effects by the wavelength dependency or the incident angle dependency of the transmittance become large.
When the transmittance depends on the incident angle, the transmittances of the mirror 3 are different from each other at the center and the outer side of the laser beam. In FIG. 7, for example, if the transmittance for the outer beam Z1 is high and the transmittance for the outer beam Z2 is low, a transmitted light quantity of the outer beam Z1 increases and a transmitted light quantity of the outer beam Z2 decreases. Therefore, intensity of an optical spot received by the light receiving section 8 is maximized at a position shifted to the Z1 side from a light receiving point of the central beam Z0. On the other hand, in FIG. 7, if the transmittance for the outer beam Z1 is low and the transmittance for the outer beam Z2 is high, a transmitted light quantity of the outer beam Z1 decreases and a transmitted light quantity of the outer beam Z2 increases. Therefore, intensity of an optical spot received by the light receiving section 8 is maximized at a position shifted to the Z2 side from the light receiving point of the central beam Z0. FIG. 8 shows optical intensity distribution (Gaussian distribution) on a light receiving surface of the light receiving section 8. Shift of the optical intensity distribution occurs as shown by a broken line according to the difference in transmittance due to the incident angle described above.
While only the zero-dimensional beam is shown in FIG. 7, but practically, the +1 dimensional beam and the −1 dimensional beam for servo control, diffracted by a diffractive optics exist in addition to the zero-dimensional beam Z. Since these diffraction one-dimensional beams exist on opposite sides of the zero-dimensional beam Z, the transmittance of the mirror 3 for the diffraction one-dimensional beam varies dependent on the incident angle as in the outer beam Z1 and the outer beam Z2 with respect to the central beam Z0 of the zero-dimensional beam. Therefore, when tracking error signals are detected based on the diffraction one-dimensional beam, there is a problem that, if shift of the optical intensity distribution occurs due to the variation in transmittance, an offset is generated in the tracking error signal and accuracy of tracking control is deteriorated. Thus, it becomes necessary to design an optical system in which stable received light signals can be obtained by suppressing displacement of the optical intensity distribution resulting from the incident angle dependency of the transmittance. However, any tactic for solving the above-mentioned problem is not described in aforesaid Japanese Unexamined Patent Publications No. 2004-139709 and No. 2004-103135.